Cores for use in precision investment casting

ABSTRACT

Concepts for fabricating improved cores for investment casting are described. The cores are composite which include refractory metal elements and ceramic elements. The refractory metal elements are provided to enhance the mechanical properties of the core and/or to permit the fabrication of cores having shapes and geometries that could not otherwise be achieved. In one embodiment, the entire core may be made of refractory metal components. The cores may be used to investment cast gas turbine superalloy components.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to investment casting cores, and inparticular to investment casting cores which are formed at least in partfrom refractory metals.

[0003] 2. Background Information

[0004] Investment casting is a commonly used technique for formingmetallic components having complex geometries, especially hollowcomponents, and is used in the fabrication of superalloy gas turbineengine components. The invention will be described in respect to theproduction of superalloy castings, however it will be understood thatthe invention is not so limited.

[0005] Gas turbine engines are widely used in aircraft propulsion,electric power generation, and ship propulsion. In all gas turbineengine applications, efficiency is a prime objective.

[0006] Improved gas turbine engine efficiency can be obtained byoperating at higher temperatures, however current operating temperaturesare at such a level that, in the turbine section, the superalloymaterials used have limited mechanical properties. Consequently, it is ageneral practice to provide air cooling for components in the hottestportions of gas turbine engines, typically in the turbine section.Cooling is provided by flowing relatively cool air from the compressorsection of the engine through passages in the turbine components to becooled. It will be appreciated that cooling comes with an associatedcost in engine efficiency, consequently, there is a strong desire toprovide enhanced specific cooling, maximizing the amount of coolingbenefit obtained from a given amount of cooling air.

[0007] Referring to FIG. 1, a gas turbine engine 10 includes acompressor 12, a combustor 14, and a turbine 16. Air 18 flows axiallythrough the sections 12, 14, and 16 of the engine 10. As is well knownin the art, air 18, compressed in the compressor 12, is mixed with fuelwhich is burned in the combustor 14 and expanded in the turbine 16,thereby rotating the turbine 16 and driving the compressor 12.

[0008] Both the compressor 12 and the turbine 16 are comprised ofrotating and stationary airfoils 20, 22, respectively. The airfoils,especially those disposed in the turbine 16, are subjected to repetitivethermal cycling under widely ranging temperatures and pressures. Toavoid thermal damage to the airfoils, each airfoil 20 includes internalcooling.

[0009] Referring to FIG. 2, the airfoil 20 includes a leading edge 26and a trailing edge 28 extending from a root end 30 to a tip 32 thereofand a platform 34. A leading edge cooling passage 40 is formed withinthe leading edge 26 of the airfoil 20 having radially extending,connected channels 42-44 and a leading edge inlet 46, formed within theplatform 34 and in fluid communication with the channel 42. A pluralityof leading edge crossover holes 48 formed within a leading edge passagewall 50 separating the channel 4 from a leading edge exhaust passage 52,allow the cooling air from the channel 44 to flow into the leading edgeexhaust passage 52. A trailing edge cooling passage 56 is formed withinthe trailing edge 28 of the airfoil 20 having radially extending,connected channels 58-60 and a trailing edge inlet 62 formed within theplatform 34 and in fluid communication with the channel 58. A firstplurality of trailing edge crossover holes 66 is formed within a firsttrailing edge wall 68 and a second plurality of trailing edge crossoverholes 72 is formed within a second trailing edge wall 74 to allowcooling air from channel 58 to flow through an intermediate passage 78to a plurality of trailing edge slots 80.

[0010] A ceramic core 120, as depicted in FIGS. 3 and 4, is used in themanufacturing process of the airfoils 20 and defines the hollow cavitiestherein. A ceramic core leading edge 126 and a ceramic core trailingedge 128 correspond to the leading edge 26 and trailing edge 28 in theairfoil 20, respectively. A ceramic core root 130 and a tip 132correspond to the airfoil root 30 and tip 32, respectively. Ceramic corepassages 140, 156 with channels 142-144, 158-160, and inlets 146, 162respectively, correspond to passages 40, 56 with channels 42-44, 58-60and inlets 46, 62, of the airfoil, respectively. Passages 52 and 78 ofthe airfoil correspond to channels 152 and 178 in the ceramic core.Pluralities of fingers 148, 166, 172 in the core 120 correspond to theplurality of crossover holes 48, 66, 72 in the airfoil 20, respectively.A core tip 190 is attached to the core passages 140, 156 by means offingers 182-185, to stabilize the core 120 at the tip 132. An externalceramic handle 194 is attached at the core trailing edge 128 forhandling purposes. A core extension 196 defines a cooling passage at theroot to the airfoil 20. Centerlines 197-199 extend radially through eachrow of fingers 148, 166, 172, respectively.

[0011] While turbine blades and vanes are some of the most importantcomponents that are cooled, other components such as combustion chambersand blade outer air seals also require cooling, and the invention hasapplication to all cooled turbine hardware, and in fact to all complexcast articles.

[0012] Currently cores such as that shown in FIGS. 3 and 4 arefabricated from ceramic materials but such ceramic cores are fragile,especially the advanced cores used to fabricate small intricate coolingpassages in advanced hardware. Current ceramic cores are prone towarpage and fracture during fabrication and during casting. In someadvanced experimental blade designs casting yields of less than 10% areachieved, principally because of core failure.

[0013] Conventional ceramic cores are produced by a molding processusing a ceramic slurry and a shaped die; both injection molding andtransfer-molding techniques may be employed. The pattern material ismost commonly wax although plastics, low melting-point metals, andorganic compounds such as urea, have also been employed. The shell moldis formed using a colloidal silica binder to bind together ceramicparticles which may be alumina, silica, zirconia and alumina silicates.

[0014] The investment casting process to produce a turbine blade, usinga ceramic core, will be explained briefly here. A ceramic core havingthe geometry desired for the internal cooling passages is placed in ametal die whose walls surround but are generally spaced away from thecore. The die is filled with a disposable pattern material such as wax.The die is removed leaving the ceramic core embedded in a wax pattern.The outer shell mold is then formed about the wax pattern by dipping thepattern in a ceramic slurry and then applying larger, dry ceramicparticles to the slurry. This process is termed stuccoing. The stuccoedwax pattern, containing the core, is then dried and the stuccoingprocess repeated to provide the desired shell mold wall thickness. Atthis point the mold is thoroughly dried and heated to an elevatedtemperature to remove the wax material and strengthen the ceramicmaterial.

[0015] The result is a ceramic mold containing a ceramic core which incombination define a mold cavity. It will be understood that theexterior of the core defines the passageway to be formed in the castingand the interior of the shell mold defines the external dimensions ofthe superalloy casting to be made. The core and shell may also definecasting portions such as gates and risers which are necessary for thecasting process but are not a part of the finished cast component.

[0016] After the removal of the wax, molten superalloy material ispoured into the cavity defined by the shell mold and core assembly andsolidified. The mold and core are than removed from the superalloycasting by a combination of mechanical and chemical means.

[0017] As previously noted, the currently used ceramic cores limitcasting designs because of their fragility and because cores withdimensions of less than about 0.012-0.015 inches cannot currently beproduced with acceptable casting yields.

[0018] Accordingly, it is an object of this invention to provide coresfor investment casting which have improved mechanical properties.

[0019] It is another object of the invention to provide cores which canbe made in thinner thicknesses than current ceramic cores.

[0020] It is another object of the invention to provide cores which areresistant to thermal shock during the casting process.

[0021] It is another object of the invention to provide cores which havegeometries and features which cannot be achieved in ceramic cores.

[0022] It is another object of the invention to provide cores whichallow rapid implementation of complex design changes without the need toemploy costly tooling and processes.

DISCLOSURE OF INVENTION

[0023] To achieve the foregoing objectives and to provide otherbenefits, in accordance with the present invention, cores are describedwhich include refractory metal elements.

[0024] Refractory metals include molybdenum, tantalum, niobium,tungsten, and alloys thereof. For purposes of this invention, the term“refractory metals” will also be understood to include intermetalliccompounds based on the foregoing refractory metals.

[0025] According to one embodiment of the invention, wires of theserefractory metals are embedded in ceramic cores to provide improvedmechanical properties.

[0026] In accordance with another embodiment of the invention, a ceramiccore may be formed about a sheet of refractory material which haspreviously been cut and shaped to conform to at least a portion of therequired core geometry.

[0027] In accordance with another embodiment of the invention arefractory wire or sheet metal element may form a portion of a core andmay be exposed to the molten metal during the casting process.

[0028] In accordance with embodiments of the invention, the refractorymetal core components may be coated with one or more layers ofprotective material to prevent the refractory constituents frominteracting with the molten metal during casting.

[0029] In accordance with another embodiment of the invention,investment casting cores may be fabricated from multiple ceramic andrefractory metal components.

[0030] The present invention may be understood by reference to thefollowing drawings taken with the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is a simplified, broken away elevation of a gas turbineengine;

[0032]FIG. 2 is an enlarged, cross-sectional elevation of an airfoil ofthe gas turbine engine of FIG. 1;

[0033]FIG. 3 is an elevation of a ceramic core defining cooling passagesfor manufacturing of the airfoil of FIG. 2 according to the presentinvention; and

[0034]FIG. 4 is a cross-sectional elevation of the ceramic core taken inthe direction of 4-4 in FIG. 3.

[0035]FIG. 5 shows a cross sectional elevation of a ceramic core takenin the direction of 4-4, illustrating embodiments of the invention

[0036]FIG. 6 shows mechanical attachment schemes;

[0037]FIG. 7 shows a refractory metal core detail for forming aconvoluted cooling passage.

DETAILED DESCRIPTION OF THE INVENTION

[0038] As previously noted, conventional ceramic cores are currently alimiting factor in the design of advanced complex superalloy articlesbecause they impose dimensional limitations on casting design. FIG. 5illustrates various embodiments of the present invention. FIG. 5 showsthe cross sectional elevation as in FIG. 4 with various illustrativerefractor metal elements.

[0039] Referring now to FIG. 5 which illustrates embodiments of theinvention, one or more refractory metal wires 200 may be embedded withinthe ceramic core to provide strength and resistance to cracking andwarping. Although shown as circular in cross section, other wire crosssections may be employed.

[0040] Wire 202 may also be located adjacent the surface ceramic of core120 and may provide a core surface contour.

[0041] Refractory metal sheet elements may also be utilized. Refractorymetal sheet elements 204 may be located at the surface of a coreelement; or a shaped refractory sheet element 206 may be shaped to forma radius and corner of a core element; similarly, a refractory metalelement 208 may form three sides and two corners of a ceramic coreelement. Refractory sheet metal element 210 may be located largelywithin a core element, extending from one surface to another, orrefractory core element 212 may be located entirely within a coreelement.

[0042] The trailing edge 128 or any one or more core elements of thecore 120 may be formed entirely from a refractory metal sheet to providea thinner core element with usable properties than could otherwise beproduced from ceramic.

[0043] Core elements or entire cores may also be built up from multipleshaped sheets 216 of refractory metals joined using various methodsincluding resistance welding, T1G welding, brazing, and diffusionbonding.

[0044] The previously described embodiments are illustrative. The coredesigner may use any one or more of these embodiments in a core design,utilizing them as appropriate in view of the specific core design.

[0045]FIG. 6 shows how a thin refractory sheet metal trailing edge corecomponent can be used to form a part of an overall investment castingcore. The thin refractory metal element 220 can be attached to theceramic portion 222 by providing a refractory metal component withregions 224 which protrude or recessed pockets 226 injecting the ceramicaround this protruding element, and/or into the pockets to provide amechanical lock between the ceramic element and the refractory metalelement.

[0046]FIG. 7 illustrates how refractory metal core elements 230 can beused to form small diameter cooling holes within the wall of an airfoil.In FIG. 10, refractory element 300 extends between the core 232 and theshell 234. Refractory element 220 will form a convoluted cooling passagein the wall of a turbine component, a cooling passage which could not beformed by casting using conventional core technology.

[0047] The refractory alloys of Mo, Cb, Ta and W are commerciallyavailable in standard shapes such as wire and sheet which can be cut asneeded to form cores using processes such as laser cutting, shearing,piercing and photo etching. The cut shapes can be deformed by bendingand twisting. The standard shapes can be corrugated or dimpled toproduce passages which induce turbulent airflow. Holes can be punchedinto sheet to produce posts or turning vanes in passageways.

[0048] Refractory metals are generally prone to oxidize at elevatedtemperatures and are also somewhat soluble in molten superalloys.Accordingly, refractory metal cores require a protective coating toprevent oxidation and erosion by molten metal. Refractory metal coreelements can be coated with one or more thin continuous adherent ceramiclayers for protection. Suitable ceramics include silica, alumina,zirconia, chromia, mullite and hafnia. Preferably, the coefficient ofthermal expansion (C.T.E.) of the refractory metal and the ceramic aresimilar. Ceramic layers may be applied by CVD, PVD, electrophoresis, andsol gel techniques.

[0049] Multiple layers of different ceramics may be employed. Individuallayers will typically be 0.1 to 1 mil thick.

[0050] Metallic layers of Pt, other noble metals, Cr and Al may beapplied to the refractory metal elements for oxidation protection, incombination with a ceramic coating for protection from molten metalerosion.

[0051] Refractory metal alloys and intermetallics such as Mo alloys andMoSi2, respectively, which form protective SiO2 layers may also bepreferred. Such materials are expected to allow good adherence of anon-reactive oxide such as alumina. It is understood that silica thoughan oxide is very reactive in the presence of nickel based alloys andmust be coated with a thin layer of other non-reactive oxide. However,by the same token silica readily diffusion bonds with other oxides suchas alumina forming mullite.

[0052] For purposes of the invention, metals containing solid solutionstrengtheners, precipitation strengtheners and dispersion strengthenersare classed as alloys.

[0053] Alloys of Mo include TZM (0.5% Ti, 0.08%2r, 0.04% C, bal Mo), andlanthanated Molybdenum Alloys of W include W-38% Re.

[0054] The previously noted alloys are by way of example and are notintended to be limiting.

[0055] After the casting process is complete the shell and core areremoved. The shell is external and can be removed by mechanical means tobreak the ceramic away from the casting, followed as necessary bychemical means usually involving immersion in a caustic solution.

[0056] In the prior art, ceramic cores are usually removed using causticsolutions, often under conditions of elevated temperatures and pressuresin an autoclave.

[0057] To the extent that the invention cores are partially ceramic, thesame caustic solution core removal techniques may be employed.

[0058] The refractory metal portion of the invention cores may beremoved from superalloy castings by acid treatments. For example, toremove Mo cores from a nickel superalloy, we have used 40 parts HNO₃ 30parts H₂SO₄, bal H₂O at temperatures of 60-100° C.

[0059] For refractory metal cores of relatively large cross sectionaldimensions thermal oxidation can be used to remove Mo which forms avolatile oxide. In Mo cores of small cross sections, we have foundthermal oxidation to be ineffective.

[0060] As noted, cores based on the metals Mo, Nb, W and Te and alloysthereof, along with intermetallic compounds based on these metals arepreferred.

1. A composite core for use in an investment casting process to producean internal passage in an investment casting which comprises a) aceramic element b) a refractory metal element attached to said ceramicelement,
 2. A composite core as in claim 1 wherein said ceramic elementis an oxide ceramic.
 3. A composite core as in claim 1 wherein saidrefractory metal element is coated with at least one oxidation resistantcoating layer.
 4. A composite core as in claim 1 wherein said refractorymetal element comprises at least one wire.
 5. A composite core as inclaim 1 wherein said refractory metal element comprises at least onesheet.
 6. A composite core as in claim 1 wherein said refractory metalelement is embedded in said ceramic element.
 7. A composite core as inclaim 1 wherein said refractory metal element is attached to the surfaceof the ceramic element.
 8. A composite core as in claim 7 wherein saidattachment is a mechanical attachment.
 9. A composite core as in claim 7wherein said attachment is a chemical bond.
 10. A mold-core assemblyuseful in the production of investment castings, having internalpassages, which comprises a) a composite core assembly including a) aceramic element b) a refractory metal element attached to said ceramicelement, the external contour of the combined ceramic and refractorymetal elements corresponding essentially to the contour of the desiredpredetermined internal passage along with gating and feeding elements.c) a ceramic shell mold surrounding said core, and spaced away from saidcore to define a cavity d) means in said shell mold for filling saidcavity with molten metal.
 11. A. east article which comprises a castsuperalloy body which contains a composite core captured within saidsuperalloy body, said composite core comprising a) a ceramic element b)a refractory metal element attached to said ceramic element, theexternal contour of the combined ceramic elements correspondingessentially to the contour of the desired predetermined internal passagealong with gating and feeding elements.
 12. A composite core for use ininvestment castings of superalloys, comprising at least one ceramicelement and at least one refractory metal element, said core having atleast a element having a dimension of less than about 0.015 inches. 13.An as cast superalloy article having an internal passage having adimension of less than about 0.012 inches.
 14. A core producing asuperalloy investment casting including a refractory metal elementforming the trailing edge of said core.
 15. A core and shell assemblyfor use in producing a superalloy investment casting including at leastone refractory metal element attached to said core and to said shell.